When trying to find his way in the dark or in tubid water, a diver needs a compass. The present-day diver's compasses are attachable to the wrist.
Under water the diver proceeds normally so that his arms are directed either along with the body straight rearwards or extended straight forwards. When the diver looks at the compass, he has to bend his arm to the proximity of his eyes. This movement may, however, cause a slight deviation from the desired direction of progress. When the compass has to be looked at several times, e.g., when a relatively long distance must be travelled without any fixed point, the total error resulting from these deviations may become fatal.
German Laid Open Publication No. 1,140,099 discloses a diver's mask having a compass and lens mounted on the inside surface of the glass. The Publication also suggests that the compass can be placed on the outside surface but in that case the lense is in any case placed inside the mask.